The present invention relates to snapshots of file systems in data storage systems.
This application also incorporates by reference herein as follows:    U.S. application Ser. No. 10/264,603, Systems and Methods of Multiple Access Paths to Single Ported Storage Devices, filed on Oct. 3, 2002, now abandoned;    U.S. application Ser. No. 10/354,797, Methods and Systems of Host Caching, filed on Jan. 29, 2003, now U.S. Pat. No. 6,965,979 B2;    U.S. application Ser. No. 10/397,610, Methods and Systems for Management of System Metadata, filed on Mar. 26, 2003, now U.S. Pat. No. 7,216,253 B2;    U.S. application Ser. No. 10/440,347, Methods and Systems of Cache Memory Management and Snapshot Operations, filed on May 16, 2003, now U.S. Pat. No. 7,124,243 B2;    U.S. application Ser. No. 10/600,417, Systems and Methods of Data Migration in Snapshot Operations, filed on Jun. 19, 2003, now U.S. Pat. No. 7,136,974 B2;    U.S. application Ser. No. 10/677,560, Systems and Methods of Multiple Access Paths to Single Ported Storage Devices, filed on Oct. 1, 2003, now abandoned;    U.S. application Ser. No. 10/696,327, Data Replication in Data Storage Systems, filed on Oct. 28, 2003, now U.S. Pat. No. 7,143,122 B2;    U.S. application Ser. No. 10/837,322, Guided Configuration of Data Storage Systems, filed on Apr. 30, 2004, now U.S. Pat. No. 7,216,192 B2;    U.S. application Ser. No. 10/975,290, Staggered Writing for Data Storage Systems, filed on Oct. 27, 2004, now U.S. Pat. No. 7,380,157 B2;    U.S. application Ser. No. 10/976,430, Management of I/O Operations in Data Storage Systems, filed on Oct. 29, 2004, now U.S. Pat. No. 7,222,223 B2;    U.S. application Ser. No. 11/122,495, Quality of Service for Data Storage Volumes, filed on May 4, 2005, now U.S. Pat. No. 7,418,531 B2;    U.S. application Ser. No. 11/245,718, A Multiple Quality of Service File System, filed on Oct. 8, 2005, now abandoned; and    U.S. application Ser. No. 11/408,209, Methods and Systems of Cache Memory Management and Snapshot Operations, filed on Apr. 19, 2006, now U.S. Pat. No. 7,380,059 B2.
Files exist to store information on storage devices (e.g., magnetic disks) and allow the information to be retrieved later. A file system is a collection of files and directories plus operations on them. To keep track of files, file systems have directories. A directory entry provides the information needed to find the blocks associated with a given file. Many file systems today are organized in a general hierarchy (i.e., a tree of directories) because it gives users the ability to organize their files by creating subdirectories. Each file may be specified by giving the absolute path name from the root directory to the file. Every file system contains file attributes such as each file owner and creation time and must be stored somewhere such as in a directory entry.
A snapshot of a file system will capture the content (i.e., files and directories) at an instant in time. A snapshot results in two data images: (1) the active data that an application can read and write as soon as the snapshot is created and (2) the snapshot data. Snapshots can be taken periodically, hourly, daily, or weekly or on user demand. They are useful for a variety of applications including recovery of earlier versions of a file following an unintended deletion or modification, backup, data mining, or testing of software.
The need for high data availability often requires frequent snapshots that consume resources such as memory, internal memory bandwidth, storage device capacity and the storage device bandwidth. Some important issues for snapshots of file systems is how to manage the allocation of space in the storage devices, how to keep track of the blocks of a given file, and how to make snapshots of file systems work efficiently and reliably.